1. Field of the Invention
The present invention generally relates to an intrusion detection system and, more particularly, to a passive intrusion detection system for indicating entry of an intruder into a protected volume.
2. Description of the Prior Art
Quartz crystal microbalance (QCM) instruments have been proposed to measure different phenomena such as wind velocity, direction, temperature as well as air humidity, contaminants in gas systems as well as in biological studies. Basically, in these studies quartz crystals are coated with different materials which affect the crystals' sensitivities to the phenomena to be measured. These crystals affect the frequencies of oscillators, with the changes in frequencies being measured to indicate the changes in the phenomena being measured.
There are many situations in which it is desired to detect the intrusion of an unauthorized person or animal into a protected volume, containing sensitive and/or dangerous materials, such as radioactive material or classified documents. Presently available infrared light and ultrasonic beam intrusion protection systems are quite inadequate since they can be perceived by an intruder, tampered by him and thus defeated. Also, presently existing systems produce an excessive number of false alarms and often require sensitive adjustments whenever the content in the protected volume is changed.